(1) Field of the Invention
The present invention relates to a process for preparing from a tertiary ether a corresponding tertiary olefin. More particularly, the present invention relates to a process for preparing a tertiary olefin having a high purity in a high yield from a tertiary ether by using a novel catalyst.
(2) Description of the Prior Art
Tertiary olefins have industrially been prepared from C.sub.4 -fractions containing tertiary olefins according to the sulfuric acid extraction process. However, this sulfuric acid extraction process is defective in that since the apparatus is corroded by the concentrated sulfuric acid used, an expensive material should be used for the apparatus and that a tertiary olefin is consumed for side reactions such as polymerization and hydration during the extraction with concentrated sulfuric acid. Therefore, this process is not advantageous from an economical viewpoint.
Recently, in U.S.A. and European countries, methyl-t-butyl ether (hereinafter referred to as "MTBE") is produced in large quantities as an octane number booster for gasoline, and there is a strong indication of the production of MTBE in Japan and there is a good possibility that MTBE will be available at a cost as low as that of gasoline. In this case, it is expected that a process for preparing isobutylene by decomposition of MTBE will become very advantageous over the sulfuric acid extraction process.
As one premise for realizing this process, it is necessary that the decomposition reaction of MTBE should be advanced at a high conversion (high decomposition ratio) with a high selectivity, and it is preferred that isobutylene and methanol obtained as decomposition products should have a high enough purity for them to be used as industrial materials.
A tertiary olefin such as isobutylene is industrially valuable as a starting material of, for example, methyl methacrylate or a butyl rubber polymer. In the latter case, it is required that the purity of isobutylene should be especially high.
Several processes have heretofore been proposed for preparing tertiary olefins from tertiary ethers. For example, Japanese Patent Publication No. 41882/72 discloses a process in which MTBE is decomposed by using a .gamma.-alumina type acidic solid catalyst having a specific surface area of at least 25 m.sup.2 /g, Japanese Patent Application Laid-Open Specification No. 39604/76 proposes a process using as a catalyst activated alumina modified by reaction with a silicon compound, Japanese Patent Application Laid-Open Specification No. 2695/80 discloses a process using a catalyst comprising silica as the main component and various metal oxides combined therewith, Japanese Patent Application Laid-Open Specification No. 94602/74 proposes a process using an active carbon catalyst, Japanese Patent Publication No. 26401/76 teaches a process using a metal sulfate as a catalyst, Japanese Patent Application Laid-Open Specification No. 85323/82 proposes a process in which an aluminum-containing silica catalyst formed, for example, by supporting aluminum sulfate on a silica carrier is used and water and/or a tertiary alcohol is added to the reaction system, Japanese Patent Application Laid-Open Specification No. 75934/82 proposes a process using a catalyst formed by supporting an aluminum compound such as aluminum sulfate on silica and heating and calcining them at a temperature higher than the decomposition temperature of the aluminum compound, Japanese Patent Application Laid-Open Specification No. 102821/82 proposes a process in which the decomposition is carried out in the presence of steam by using a catalyst comprising titanium, hafnium or zirconium supported on alumina, Japanese Patent Application Laid-Open Specification No. 123124/82 teaches a process using an acidic molecular sieve as the catalyst, Japanese Patent Application Laid-Open Specification No. 134421/82 proposes a process using a catalyst formed by supporting a metal sulfate on a carrier which has been calcined at a high temperature, and Japanese Patent Application Laid-Open Specification No. 142924/82 teaches a process using a catalyst formed by supporting an aluminum compound on a carrier containing silicon oxide and heating and calcining them at a temperature higher than the decomposition temperature of the aluminum compound.
These known processes, however, are defective in various points. For example, since an ether such as dimethyl ether is formed as a by-product by dehydration of two molecules of a methanol formed by the decomposition of MTBE, the alcohol recovery ratio is low. Furthermore, the reaction temperature is very high and the preparation of catalysts is troublesome, and expensive chemicals should be used. Moreover, the catalyst life is short and the durability is insufficient. Thus, none of the catalysts heretofore proposed are industrially satisfactory.
Since the reaction of forming a tertiary olefin by decomposition of a tertiary ether is an endothermic reaction, from the energy-saving viewpoint, it is desirable that a high decomposition ratio of the tertiary ether and a high selectivity to the tertiary olefin and alcohol should simultaneously be attained at a low temperature level.